Manufacturing method of seamless steel pipe for mechanical structural parts

ABSTRACT

The present invention provides a manufacturing method of seamless steel pipe for mechanical structural parts. According to this manufacturing method, production process is simplified and production cost is reduced by employing inline quenching, and simultaneously, the toughness of steel pipe is guaranteed by controlling Ti/N to be 3.5 or lower. A manufacturing method of seamless steel pipe, in which steel having the chemical composition: by weight percent, 0.10 to 0.25% of C, not greater than 1.00% of Si, 0.20 to 2.00% of Mn, not greater than 0.03% of P, not greater than 0.020% of S, 0.10 to 1.5% of Cr, not greater than 0.5% of Mo, 0.005 to 0.030% of Ti, 0.01 to 0.10% of V, and the balance being Fe and incidental impurities, and Ti/N being 3.5 or lower, is subjected to finish rolling at temperature of 900° C. or higher and the section-decrease rate of 40% or greater; then subjected to soaking directly at temperature of 900 to 1000° C. without cooling, and then quenching and tempering.

TECHNICAL FIELD

The present invention relates to a manufacturing method of seamlesssteel pipe, especially a manufacturing method of seamless steel pipe formechanical structural parts.

BACKGROUND ART

Conventionally, in order to ensure the strength and toughness of grade60K and 80K steel pipe used for cylinders, offline quenching process isemployed hitherto. However, it causes long production period and highmanufacturing cost.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a manufacturing methodof seamless steel pipe for mechanical structural parts. According tothis manufacturing method, production process is simplified andproduction cost is reduced by employing inline quenching, andsimultaneously, the toughness of steel pipe is guaranteed by controllingTi/N to be 3.5 or lower. In order to obtain this object, themanufacturing method of seamless steel pipe for mechanical structuralparts according to the present has the following aspects.

(1) A manufacturing method of seamless steel pipe, in which, steelhaving the chemical composition: by weight percent, 0.10 to 0.25% of C,not greater than 1.00% of Si, 0.20 to 2.00% of Mn, not greater than0.03% of P, not greater than 0.020% of S, 0.10 to 1.5% of Cr, notgreater than 0.5% of Mo, 0.005 to 0.030% of Ti, 0.01 to 0.10% of V, andthe balance being Fe and inevitable impurities, and Ti/N being 3.5 orlower, is subjected to finish rolling at temperature of 900° C. orhigher and the section-decrease rate of 40% or greater; then subjectedto soaking directly at temperature of 900 to 1000° C. without cooling,and then quenching and tempering.

(2) A manufacturing method of seamless steel pipe according to (1),wherein the chemical composition of the steel used therein furthercomprises one or more components selected from C: not greater than 0.5%,Ni: not greater than 1.0% and Nb: not greater than 0.01%.

Reasons for limiting the chemical composition of the steel used in themanufacturing method of seamless steel pipe for mechanical structuralparts according to the present invention are specified below. ContentRange Reasons for Limiting C 0.10 to C is an indispensable element forensuring 0.25% quenching. However, if the C content is less than 0.10%,the effect is not sufficient. On the other hand, if the C contentexceeds 0.25%, crack in quenching, reduction of toughness, deteriorationof weldability and machining ability of product would happen. Therefore,the C content is defined within a range of 0.10 to 0.25%. Further, dueto the decrease of alloy elements with the strength improvement, thecontent C may be defined to 0.16% or great. Si Not greater Si is aneffective element for deoxidizing than 1.00% and improving strength ofsteel. However, if Si content exceeds 1.0%, excessive Si would makesteel fragile. In order to ensure excellent toughness, the Si content iscontrolled at not greater than 1.0%. Mn 0.20 to Mn is an indispensableelement for 2.00% deoxidizing and desulfurizing of steel, and alsoeffective for ameliorating strength and heat machining ability andobtaining suitable structure. If the Mn content is less than 0.2%, sucheffects are not sufficient. Furthermore, if the Mn content exceeds 2.0%,although the strength is improved, the weldability and machining abilitywould be deteriorated. Therefore, the Mn content is defined within arange of 0.2 to 2.0%. P Not greater P exists in steel as an inevitableimpurity. than 0.030% If the P content exceeds 0.030%, boundary ofcrystal grains will be segregated to deteriorate the toughness.Therefore, the P content is controlled at not greater than 0.030%. S Notgreater S, similar to P, exists in steel as an than 0.020% inevitableimpurity. Since S would form coarse inclusion, especially bringsdeterioration of toughness in the rolling direction and right-angle (Tdirection) of steel. Therefore, S content is controlled at not greaterthan 0.020%. Cr 0.10 to Cr, similar to C, is an indispensable element1.5% for ensuring quenching. However, If the Cr content is less than0.1%, its effect would be insufficient. On the other hand, if the Crcontent exceeds 1.5%, the weldability and machining ability would bedecreased. Therefore, the Cr content is defined within a range of 0.1 to1.5%. Mo Not greater Mo is an indispensable element for ensuring than0.5% toughness by quenching and tempering at high temperature. However,if the Mo content exceeds 0.5%, such effects become saturated, while themachining ability of pipe-making is degrade due to the segregation.Therefore, the Mo content is controlled at not greater than 0.5%. Ti0.005 to Ti is an element for refining the crystal 0.030% grains toimprove toughness. However, if the Ti content is less than 0.005%, Itseffect would be insufficient. On the other hand, if the Ti contentexceeds 0.030%, excessive addition would form coarse carbides to degradethe toughness. Therefore, the Ti content is defined within a range of0.005 to 0.030%. V 0.01 to V is an element for improving the toughness0.10% by tempering at high temperature. However, if the V content isless than 0.01%, its effect would be insufficient. On the other hand, ifthe V content exceeds 0.10%, the excessive addition would form coarsecarbides to degrade the toughness. Therefore, the V content is definedwithin a range of 0.01 to 0.10%. Ti/N Not greater The excessive additionof Ti would form TiC than 3.5 precipitated to deteriorate steel. TiNformed by the combination of Ti and N is effective to suppress theprecipitation of TiC. Therefore, if the atomic weight ratio of Ti/N iscontrolled at not greater than 3.5, the precipitation of TiC can besuppressed.

By limiting the chemical composition of the steel used in the presentinvention to the above-stated ranges, the seamless steel pipe formechanical structural parts with refined crystal grains and excellenttoughness and strength can be obtained. If further improvements ofcrystal grain refinement, toughness and strength of steel are required,one or more elements of Ni, Cu and Nb may be added to the chemicalcomposition described above. The reasons for limiting the addition ofthese elements are described as follows. Ni Not greater Ni is aneffective element for ameliorating than 1.0% quenching and improvingtoughness. However, at the viewpoint of cost, the Ni is an expensivealloy element, so that the Ni content is controlled at not greater than1.0%. Cu Not greater Cu is an effective element for improving the than0.5% strength and corrosion resistance. However, if the Cu contentexceeds 0.5%, the surface on a steel pipe would often generate defects.Therefore, the Cu content is controlled at not greater than 0.5%. Nb Notgreater Nb, similar to Ti, is an effective element for than 0.01%refining the crystal grains and improving toughness. However, in thein-line heat treatment process, the uneven distribution of precipitationof Nb would cause the strength unevenness of the product. Therefore, theNb content is controlled at not greater than 0.01%.

In the present invention, the billet heating temperature is notspecifically defined provided that it enables the hot piercing by apiercer. The optimal temperature is determined in accordance with thevariety of steels, high temperature ductility and high temperaturestrength. Generally, the billet is heated at a range of 1100 to 1300° C.The piercing step is a process using a piercer to make a raw hollow pipeby piercing a solid billet. In order to ensure the finish rollingcomprising of stretching and sizing to be carried out easily, crosspiercer by a coniform roller is employed in this step.

In the present invention, when subjecting the raw hollow pipe to thefinish rolling (which is composed of stretching and sizing), thesection-decrease rate is controlled to 40% or greater, and thetemperature at 900° C. or higher. If the section-decrease rate is lessthan 40%, the recrystallization cannot be performed to realizerefinement effect of crystal grains. Meanwhile, the crystal grains wouldsometimes grow abnormally. The upper limit of the section-decrease ratein the finish rolling is hard to define since it varies depending on thematerials for pipe-making and the capability of rollers. However, sincean excessive section-decrease rate would readily cause defects, itsupper limit is preferably controlled at 80%.

The machining temperature in finish rolling varies according to thematerials for steel pipe and the rollers. However, if the temperature islower than 900° C., the deformation impedance of steel becomes larger,thus making the refining processing (finish rolling) with thesection-decrease rate at 40% or greater difficult. Therefore, thetemperature is defined to not lower than 900° C. Although the upperlimit of the finish temperature is hard to be defined since it dependson the materials for steel pipe and rollers, it is still preferablydefined to 1100° C.

A character of the present invention is that, the steel pipe is notcooled between the finish rolling and heat treatment of quenching andtempering but directly subjected to recrystallization treatment(normalizing). Thereby, recrystallization is induced by the combinationof the machining and heat treatment and thus realizing the grain-sizingof the grains.

In the prior art, reheating process is necessary betweenstretch-processing and sizing-processing in rolling. But, according tothe present invention, the machining process is not necessary aftersoaking, so that the soaking temperature can be set at the lowesttemperature enabling the recrystallization, and thereby obtaining thesized recrystal grains. Since the present invention uses Cr—Mo steel, ifthe soaking temperature is less than 900° C., the time needed forrecrystallization would be long, and the pipe-making efficiency would beremarkably degraded. On the other hand, if the soaking temperatureexceeds 1000° C., the excessive refinement of crystal grains and declineof the toughness become the cause of cracking in second machining.Therefore, the soaking temperature is defined within a range of 900 to1000° C. After soaking, direct quenching is carried out.

In the prior art that carries out offline quenching, the temperature ofsteel pipe is increased from room temperature, so that the steel pipeshould stays in a heating furnace for a long time. Therefore, it is noteconomic. Furthermore, the scale would grow greatly on the surface ofthe product, thus requiring a removing step by acid wash or shot blastaccording to the uses.

According to the manufacturing method of the present invention, sincethe refining process of steel pipe is carried out at temperature of 900°C. or higher, the steel pipe can be soaked directly in a reheatingfurnace. Therefore, the remaining time in the furnace can be controlledat not greater than 30 minutes, thereby it is economic in energyexpense.

In order to ensure the objective strength, the tempering in the presentinvention is carried out at a predetermined temperature. Since V isadded to the chemical composition of Cr—Mo steel, VC would precipitateat temperature of 500 to 600° C., and thus causing the decline of thetoughness. Therefore, the tempering process is carried out generally attemperature of 620 to 720° C.

EXAMPLES

Steels marked as A to M having chemical compositions shown in Table 1were, with conventional methods, melted, cut into blocks and rolled toobtain billets each with a diameter of 225 mm. Each billet was heated at1250° C. and then pierced by a piercer to form raw hollow pipes.

The raw hollow pipe made from the steels marked as A to G with thechemical components according to the present invention were subjected tofinish rolling composed of stretching and sizing under conditions shownin Table 2 to obtain seamless steel pipes each with an outer diameter of240 mm and a wall thickness of 8 to 30 mm. Then, without being cooled,the seamless steel pipes thus obtained were soaked directly undercondition shown in Table 2, then subjected to heat treatment byquenching and tempering, thus obtaining examples 1 to 7 shown in Table2.

Further, the raw hollow pipes made from the steels marked with H to M,having the chemical composition contents beyond the ranges prescribed inthe present invention, were subjected to finish rolling composed ofstretching and sizing under conditions shown in Table 2 to obtainseamless steel pipes each with an outer diameter of 240 mm and a wallthickness of 8 to 30 mm. Then, without being cooled, the seamless steelpipes thus obtained were soaked directly, and then subjected to heattreatment by quenching and tempering under the conditions shown in Table2 within the range prescribed in the present invention, thus obtainingcomparative examples 1 to 6 shown in Table 2.

Furthermore, the raw hollow pipes made from steels marked with A,C,D andF were subjected to finish rolling composed of stretching and sizingunder conditions shown in Table 2 to obtain seamless steel pipes eachwith an outer diameter of 240 mm and a wall thickness of 8 to 30 mm.Then, without being cooled, the seamless steel pipes thus obtained weresoaked directly, and then subjected to heat treatment by quenching andtempering under the conditions shown in Table 2 beyond the rangeprescribed in the present invention, thus obtaining comparative examples7 to 10 shown in Table 2.

Furthermore, the hollow raw pipes made for the steels marked as B, E andG were subjected to finish rolling by stretching and sizing to produceseamless steel pipes each with an outer diameter of 240 mm and a wallthickness of 8 to 30 mm under conditions shown in Table 2. Each of theseamless steel pipes thus obtained was once cooled to room temperaturein accordance with the prior art, then heated in a quenching furnaceunder the condition shown in Table 2, and then subjected to heattreatment of water quenching and tempering, thus obtaining the prior artexamples 1 to 3 shown in Table 2.

In the column of soaking temperature in Table 2, the temperature inbrackets( ) represents the soaking temperature of the steel pipeincreased in quenching furnace after its being finish rolled and cooledto room temperature.

Evaluation on each seamless steel pipe is described as follows.Mechanical performances were measured using 12 C test piece prescribedin JISZ 2201 metal material tensile test piece, and the tensile test wascarried out using the metal material tensile test according to JISZ2241. TS≧590 MPa and YS≧490 MPa were target values. Furthermore,toughness was measured using V-type notch test piece with a width of 10mm prescribed in JISZ 2202 metal material impact test piece, and theCharpy Impact test was carried out using metal material impact testprescribed in JISZ 2242. toughness≧100 was the target value. Testresults are shown in Table 2.

It can be known from Table 2 that seamless steel pipes of comparativeexamples 1 to 10 cannot achieve the target values of strength and/ortoughness.

Furthermore, although the seamless steel pipes of the prior art examplesreach the target values of strength and toughness, due to the offlinequenching process employed therein, the steel pipe is once cooled to theroom temperature and then increased from room temperature to a highertemperature, so that the steel pipe should stays in a heating furnacefor a long time. Therefore, it is not economic. Furthermore, the scalewould grow greatly on the surface of the product, thus requiring aremoving step by acid wash or shot blast according to the uses of thesteel pipes. Therefore, comparing to the present invention, the priorart has the problem of longer production period and higher productioncost.

According to the manufacturing method of seamless steel pipe formechanical structural parts of the present invention, the billet isheated, pierced and rolled, then finish rolled by stretching and sizingat temperature of 900° C. or higher with section-decrease rate of 40% orhigher. This process realizes great machining deformation. Furthermore,after finish rolled, the steel pipe is soaked directly at temperature of900 to 1000° C. without cooled, then quenched inline, and then kept at apredetermined temperature and then subjected to tempering so as to reacha desirable strength. Due to this process, the product manufactured byinline quenching is ensured to have performances equivalent to those ofthe product manufactured by offline quenching in the prior art.Therefore, compared to the manufacturing method of prior art, themanufacturing method according to the present invention can achieve theeffects of simplifying manufacturing process, improving pipe-makingefficiency and saving energy, and producing seamless steel pipe formechanical structural parts with excellent toughness at lower cost.TABLE 1 C Si Mn P S Cr Mo Ti V N Ti/N Ni Cu Nb Remarks Steels A 0.140.27 1.45 0.014 0.004 0.23 0.01 0.015 0.05 0.0054 2.78 used B 0.11 0.241.43 0.015 0.004 0.22 0.01 0.018 0.05 0.0052 3.46 60Q in the C 0.14 0.271.33 0.015 0.004 0.20 0.01 0.012 0.05 0.0106 1.13 0.02 Present D 0.190.24 0.94 0.010 0.007 0.53 0.21 0.018 0.06 0.0055 3.27 0.001 Invention E0.18 0.21 0.94 0.009 0.007 0.52 0.19 0.013 0.05 0.0050 2.60 0.02 80QA F0.14 0.23 0.78 0.020 0.003 0.48 0.33 0.016 0.03 0.0046 3.48 0.76 0.1680QC G 0.13 0.25 0.80 0.017 0.003 0.46 0.31 0.019 0.04 0.0055 3.45 0.690.16 0.002 Steels H 0.14 0.31 1.46 0.010 0.004 0.22 0.01 0.020 0.060.0035 5.71 60Q used I 0.14 0.19 0.85 0.012 0.003 0.50 0.35 0.026 0.040.0033 7.88 0.71 0.17 0.002 80QA in J 0.20 0.25 0.96 0.012 0.006 0.540.21 0.027 0.05 0.0041 6.59 0.001 80QC Comparative K 0.06 0.25 0.800.015 0.004 0.23 0.01 0.025 0.05 0.0054 4.63 Invention L 0.30 0.25 1.330.015 0.004 0.23 0.01 0.025 0.05 0.0050 5.00 M 0.18 0.21 0.94 0.0090.007 0.52 0.01 0.040 0.05 0.0050 8.00

TABLE 2 Section decrease Finish Rolling Soaking Tempering Steel Sort Infinish rolling Temp. Temp. Temp. TS YS vE0° C.(J) Value Examples 1Steels used A 52 1000 950 650 776 699 224 185 201 ∘ of the 2 in presentB 52 1040 950 650 685 585 217 180 186 ∘ present 3 invention C 52 1045950 660 684 582 193 193 191 ∘ invention 4 D 52 1000 950 650 916 844 153140 149 ∘ 5 E 52 910 950 650 904 828 168 163 179 ∘ 6 F 52 1000 950 650881 751 187 187 185 ∘ 7 G 52 950 950 650 887 831 195 170 183 ∘Comparative 1 Steels used H 52 1020 950 650 786 719 89 91 70 x Examples2 in present I 52 970 950 650 925 861 61 73 50 x 3 invention J 52 1050950 650 938 857 64 57 66 x 4 K 52 1030 950 650 570 463 243 230 221 x

1. A manufacturing method of seamless steel pipe, in which steel havingthe chemical composition: by weight percent, 0.10 to 0.25% of C, notgreater than 1.00% of Si, 0.20 to 2.00% of Mn, not greater than 0.03% ofP, not greater than 0.020% of S, 0.10 to 1.5% of Cr, not greater than0.5% of Mo, 0.005 to 0.030% of Ti, 0.01 to 0.10% of V, and the balancebeing Fe and incidental impurities, and Ti/N being 3.5 or lower, issubjected to finish rolling at temperature of 900° C. or higher and asection-decrease rate of 40% or greater; then subjected to soakingdirectly at temperature of 900 to 1000° C. without cooling, and thenquenching and tempering.
 2. A manufacturing method of seamless steelpipe according to claim 1, wherein said steel further comprises one ormore components selected from C: not greater than 0.5%, Ni: not greaterthan 1.0% and Nb: not greater than 0.01%.